1. Field of the Invention
The present invention relates to a water-resistant casing structure for use with an electronic control device.
2. Description of the Related Art
One known water-resistant casing structure for use with an electronic control device for vehicle-mounted engine control, for example, employs a rubber gasket as disclosed in Japanese laid-open patent publication No. H10-22658.
The disclosed water-resistant casing structure will be described below with reference to FIGS. 17, 18A, and 18B of the accompanying drawings. FIG. 17 is a fragmentary cross-sectional view of a water-resistant casing structure of a casing 101 and a connector 103 attached thereto. FIGS. 18A and 18B are cross-sectional views taken along line XVIII—XVIII of FIG. 17. As shown in FIGS. 17 and 18A, the connector 103 is inserted in a joint hole 102 defined in the casing 101 with a water-resistant seal 104 placed in a gap between the connector 103 and edges of the casing 101 which define the joint hole 102, hermetically sealing the casing 101.
When the temperature of the casing 101 changes, a pressure difference is developed between the external and internal spaces of the casing 101, i.e., the difference between an external pressure Po around the casing 101 and an internal pressure Pi within the casing 101. If the casing 101 is made of a material of low strength, then the ceiling or upper edge of the casing 101 above the joint hole 102 flexes due to the pressure difference. Specifically, when the internal pressure Pi becomes lower than the external pressure Po, i.e., when a negative pressure is developed in the casing 101, the ceiling of the casing 101 above the joint hole 102 flexes so as to be convex downwardly as shown in FIG. 188. As a result, the thickness of the water-resistant seal 104 decreases from t10 to t11 (<t10). The water-resistant seal 104 undergoes greater stresses and may possibly lose its water-resistant capability. If the sealing function of the water-resistant seal 104 fails, then the electronic control device enclosed in the casing 101 may be damaged by water introduced into the casing 101.